There exist radio-telecommunication terminals having:
a radio-frequency transceiver section able to receive a radio signal transmitted on a radio-frequency channel and to transform the received radio signal into a baseband signal, the radio-frequency transceiver section having a noise cancellation module able to efficiently cancel noise on the radio-frequency channel only if the noise energy gradient remains between a predetermined lower and upper limit,
a baseband section to process the baseband signal, the baseband section having at least a digital data bus, two electronic circuits that transmit data bursts through the bus, and a bus manager able to indicate that the bus is busy as long as a data burst is being transmitted and to indicate that the bus is free for the transmission of a new data burst from the end of the previous data burst transmission, each data burst transmission generating noise of constant energy on the radio-frequency channel.
A data burst is a temporal succession of binary data to be transmitted as a single block of information. Thus, during the transmission of a data burst, the bus manager continuously indicates that the bus is busy from the beginning of the data burst transmission to the end of the data burst transmission. As a result, the transmission of a data burst cannot be interrupted to transmit another data burst from another circuit.
For example, a data burst occurs when a circuit reads or writes data in a set of consecutive addresses in a memory.
During a data burst transmission, the information is transmitted over the bus by changing bus wire levels from high to low level and vice-versa in synchronism with a bus clock. This transition from high to low level and from low to high level generates noise at frequencies that depend on the bus clock frequency. Generally, the generated noise occurs at harmonic frequencies of the bus clock frequency.
For example, in GSM (Global System for Mobile Communications) the bus between a baseband processor and an external memory is clocked at 13 MHz. Consequently, a data burst transmission generates noise at 936 MHz that is the 72th harmonic of 13 MHz. The noise at 936 MHz is within the frequency band of the radio-frequency channel at 900 MHz (this channel is known as “channel 5” in GSM mobile phones) and disrupts this channel.
More precisely, the noise generated during the transmission of data bursts is either radiated from the bus wires to an antenna connected to the radio-frequency transceiver section or conducted to the radio-frequency transceiver section through conductors like power or ground conductors. As a result, in a power density spectrum of the received radio signal, this generated noise appears as a parasitic power peak of constant amplitude or as a constant offset of the amplitude of the main power peak of the received radio signal. In this last case, the offset caused by the noise is known as “DC offset”.
As long as the energy of the generated noise does not rapidly vary with time, the noise cancellation module is able to efficiently cancel this noise.
However, the applicant has noted that the noise energy abruptly decreases at the end of the data burst transmission and abruptly increases at the beginning of the burst data transmission because the bus shifts from an idle state to a busy state and vice-versa. Therefore, the noise cancellation module is not permanently efficient to cancel the noise generated by data burst transmissions.
Many solutions have been proposed to solve this problem. For example, adjusting the bus clock frequency to offset the noise from the radio-frequency channel is disclosed in U.S. Pat. No. 5,926,514 in the name of Meador et al. It has also been proposed:
a) to reduce the noise in the baseband section, or
b) to reinforce the shielding between the baseband section and the radio-frequency transceiver section.
Solution a) is not suitable because it goes against the trend to have more powerful circuits in the baseband section.
Solution b) is not suitable either, because it goes against the trend of miniaturization.